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Destruction of Bowman's layer: cause for minimal concern during PRK
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The destruction of Bowman's layer by photorefractive keratectomy (PRK) is not a reason to discontinue the procedure, but it is cause for some concern, according to Jonathan H. Talamo, MD, clinical assistant professor of ophthalmology at Harvard Medical School and medical director of the Laser Eye Center of Boston, and Jan Bergmanson, OD, PhD, professor at the University of Houston School of Optometry.
"Patients should be told about potential complications such as epithelial erosion," Bergmanson said. "So far we have seen only minor problems. I wouldn't want to build up patients' anxiety. I would rather encourage further research to understand how Bowman's membrane functions."
Talamo added that as long as eye care practitioners make sure patients receive the mandated information packet and discuss it with them, they have done enough.
"Patients are aware that the clinical trials have only 3 years of follow-up and there are unforeseen things that can happen," he said. "They know about so-called haze or scarring, that it might happen early or late, but it more than likely won't happen at all as long as they are within the appropriate level of correction. Many European series have shown that PRK is quite safe for the vast number of patients who have undergone it."
Problems are rare
Talamo said that of the hundreds of PRKs he has performed over 6 years, he has seen only one case where the absence of Bowman's layer may have caused a problem. His patient developed epidemic keratoconjunctivitis (EKC) in one of her eyes a year after uneventful PRK. The eye showed subepithelial infiltrates and associated scarring that resulted in irregular epithelium of the cornea, irregular astigmatism and decreased best corrected vision. She recovered her vision within a year and a half but lost some PRK correction.
The scarring eventually receded and the surface of the eye eventually became more regular. Meanwhile, Talamo delayed retreatment because she kept improving with each visit.
"Although it could have happened even if she had not had PRK," Talamo said, "I could not help but wonder if the absence of Bowman's layer made her a bit more susceptible to this severe bout with EKC and vision loss. There might have been some compromise in her status, but it certainly did not blind her, it just set her back a little while."
Both Bergmanson and Talamo noted that the function of Bowman's layer is unknown. The structure, a compact layer of collagen 15 to 20 microns thick, differs in orientation from the underlying stroma. Talamo suggested that it may act as a sink for growth factors and other chemical mediators that help preserve corneal clarity and limit interactions between epithelium and stroma.
No capacity for regeneration
Bergmanson said eradication of Bowman's layer in the ablated zone during PRK may be a problem because it is largely acellular and shows no capacity for regeneration. Only primates have Bowman's layer, while other species seem to manage well without it. When it is destroyed, the foundation upon which new layers of epithelium are laid is gone.
"It will never come back, and epithelium grows back on unfamiliar ground. We have seen abnormal stromal interactions from this," he said.
He added that after PRK, the adhesion complex, which keeps the epithelium attached to the stroma, does not recover fully to its original structure. "There are focal spots where particular molecules won't come back, and it no longer completely covers the underlying area."
When the complex is deficient, more epithelium can be lost, leading to patterns in epithelium erosion. Bergmanson said that erosion typically occurs with aging, but patients may also be even more vulnerable to it after PRK. Although recovery normally is good, it is not complete, and patients will have an adhesion apparatus that is not in a perfect state.
"In all fairness, there have been minimal erosion problems reported in PRK patients," Bergmanson said. "But it is still early. Follow-up for PRK has been for only 3 years, and most people undergoing the procedure have been young."
Talamo noted that conditions including Salzmann's nodular degeneration and Reise-Buckler, averian, granular, lattice or other dystrophies, which sometimes destroy Bowman's layer, show the structure is not necessary for clarity.
However, he added, the absence of Bowman's layer could potentially open up opportunity for more interaction between corneal epithelium and stroma in wound healing. "That is certainly a theoretical concern and one factor that led people to explore lamellar surgery, such as laser in situ keratomileusis or automated lamellar keratoplasty, as opposed to surface surgery for correction of refractive errors," Talamo said.
For Your Information:
- Jan Bergmanson, OD, PhD, can be reached at the University of Houston School of Optometry, Houston, TX 77204; (713)743-1950; e-mail: Jbermanson @uh.edu.
- Jonathan H. Talamo, MD, can be reached at the Laser Eye Center of Boston, Boston, MA 02114; (617) 523-2010; fax: (617) 523-4242; e-mail: jht@tiac.net.
- A discussion of Bowman's layer and PRK can be found in The Excimer Manual chapter 1, "Basic Science and Principles." Written by Jonathan H. Talamo, MD, and Ronald R. Krueger, MD, the book is available through Little, Brown and Co., 34 Beacon St., Boston, MA 02108; (617) 227-0730.